1. Field of Invention
The present invention relates to an exhaust gas purification device of an engine.
2. Description of Related Art
In the related art, in a diesel engine, particulates contained in the exhaust gas are removed by arranging a particulate filter in the engine exhaust passage, using that particulate filter to trap the particulates in the exhaust gas, and igniting and burning the particulates trapped on the particulate filter to recycle the particulate filter. The particulates trapped on the particulate filter, however, do not ignite unless the temperature becomes as high as at least about 600° C. However, the temperature of the exhaust gas of a diesel engine is normally considerably lower than 600° C. Therefore, it is difficult to use the heat of the exhaust gas to cause the particulates trapped on the particulate filter to ignite. To use the heat of the exhaust gas to cause the particulates trapped on the particulate filter to ignite, it is necessary to lower the ignition temperature of the particulates.
It has been known in the related art, however, that the ignition temperature of particulates can be reduced if a catalyst is carried on the particulate filter. Therefore, known in the art are various particulate filters carrying catalysts for reducing the ignition temperature of the particulates.
For example, Japanese Examined Patent Publication (Kokoku) No. 7-106290 discloses a particulate filter comprising a particulate filter carrying a mixture of a platinum group metal and an alkali earth metal oxide. In this particulate filter, the particulates are ignited at a relatively low temperature of about 350° C. to 400° C. and are then continuously burned.
Summarizing the problem to be solved by the invention, in a diesel engine, when the load becomes high, the temperature of the exhaust gas reaches from 350° C. to 400° C., therefore with the above particulate filter, it would appear at first glance that the particulates could be made to ignite and burn by the heat of the exhaust gas when the engine load becomes high. In fact, however, even if the temperature of the exhaust gas reaches from 350° C. to 400° C., sometimes the particulates will not ignite. Further, even if the particulates ignite, only some of the particulates will burn and a large amount of the particulates will remain unburned.
That is, when the amount of the particulates contained in the exhaust gas is small, the amount of the particulates deposited on the particulate filter is small. At this time, if the temperature of the exhaust gas reaches from 350° C. to 400° C., the particulates on the particulate filter ignite and then are continuously burned.
If the amount of the particulates contained in the exhaust gas becomes larger, however, before the particulates deposited on the particulate filter completely burn, other particulates will deposit on that particulates. As a result, the particulates deposit in layers on the particulate filter. If the particulates deposit in layers on the particulate filter in this way, the some of the particulates easily contacting the oxygen will be burned, but the remaining particulates which do not contact the oxygen will not burn and therefore a large amount of particulates will remain unburned. Therefore, if the amount of particulates contained in the exhaust gas becomes larger, a large amount of particulates continues to deposit on the particulate filter.
On the other hand, if a large amount of particulates is deposited on the particulate filter, the deposited particulates gradually become harder to ignite and burn. It probably becomes harder to burn in this way because the carbon in the particulates changes to hard-to-burn graphite etc. while being deposited. In fact, if a large amount of particulates continues to deposit on the particulate filter, the deposited particulates will not ignite at a low temperature of 350° C. to 400° C. A high temperature of over 600° C. is required to cause ignition of the deposited particulates. In a diesel engine, however, the temperature of the exhaust gas usually never becomes a high temperature of over 600° C. Therefore, if a large amount of particulates continues to deposit on the particulate filter, it is difficult to cause ignition of the deposited particulates by the heat of the exhaust gas.
On the other hand, at this time, if it were possible to make the temperature of the exhaust gas a high temperature of over 600° C., the deposited particulates would be ignited, but another problem would occur in this case. That is, in this case, if the deposited particulates were made to ignite, it would burn while generating a luminous flame. At this time, the temperature of the particulate filter would be maintained at over 800° C. for a long time until the deposited particulates finished being burned. If the particulate filter is exposed to a high temperature of over 800° C. for a long time in this way, however, the particulate filter will deteriorate quickly and therefore the problem will arise of the particulate filter having to be frequently replaced with a new filter.